The deadly suicide attacks killed at least 38 in the heart of downtown Moscow, once again presenting Putin with both a challenge to stability and an opportunity to consolidate power.

Reporting from Moscow — The suicide bombs that roared through Moscow subway cars Monday were almost certainly the latest salvo in a slow-moving war of attrition between the Russian government and militants in the restive, mostly Muslim republics of the Caucasus.

Vladimir Putin has been trading blows with southern rebels ever since he rose to the presidency a decade ago. At times, violence has threatened to erode the social contract he's struck with the Russian public: Forgo some democratic rights in exchange for, above all, stability.

And yet, many analysts say, the war in Chechnya consolidated Putin's power, by persuading people to unite with him against the threats. The militants have both menaced and strengthened Putin's leadership, they say.

On Monday, two female suicide bombers boarded packed subway cars in bustling downtown Moscow in the middle of rush hour and blew themselves up, killing at least 38 people and injuring dozens more. It was the first such attack in Moscow in six years, and it raised the specter of violence creeping back into the heart of Russia.

The killings seemed intended to rattle the very core of Russian identity. Lubyanka Square, site of the first station to be attacked, holds a deep and unsettling place in the Russian consciousness as the headquarters of the Soviet KGB, and now its successor, the FSB.

Next came Park Kultury, another iconic station alongside Gorky Park, where Russian children gather for roller coaster rides, playtime in sprawling gardens and ice skating.

Investigators said they had identified one of the bombers and were hunting for two women seen on surveillance camera video accompanying the attackers to the doors of a subway station in southwest Moscow, law enforcement sources told Interfax news agency.

Officials recovered some of the remains of the attackers, which were sent for forensic identification. The body parts included a head believed to belong to one of the bombers, unnamed investigators told Russian news agencies.

"Probably it was a reply to some injustice or atrocity done to their fathers or brothers, whoever, but it's only the end of a tentacle," said Sergei Arutyunov, chair of the Caucasus department at the Russian Academy of Science. "And the tentacles converge in a large, loose body of separatism and pseudo-Islamic fanaticism."

There was no immediate claim of responsibility, though suspicion fell on Chechen militants, who have used women in a number of attacks.

However, some officials speculated that the blasts could be an act of vengeance from supporters of Said Buryatsky, an Islamist ideologue who was reported killed by security services this month in the republic of Ingushetia. The Russian government has blamed Buryatsky for a spate of recent attacks, including the November bombing of a high-speed train between Moscow and St. Petersburg.

An Islamist website later confirmed Buryatsky's death. Another rebel leader, Doku Umarov, threatened Russian cities in a February interview with a website linked to the Islamists. "Blood will no longer be limited to our cities and towns," Umarov said. "The war is coming to their cities."

The Metro explosions occurred a few days after the 10th anniversary of Putin's election to the presidency. In 2008, because of term limits, Putin was forced to give up that office, and he now serves as prime minister. But he is widely seen as Russia's ultimate authority, and many analysts expect him to return to the presidency in the next elections.

Much of Putin's time in power has been defined by the struggle with Islamic militants in the Caucasus.

Putin was elevated to national power by President Boris N. Yeltsin, who had fought a disastrous campaign in Chechnya. Putin returned Chechnya to Moscow's control through a second war. After the installation of proxy leadership to crack down on separatists and lingering, heavy-handed efforts to quash violence, the bloodshed has resurged on the southern edge of Russia -- and raised questions about the government's ability to stabilize the country.

Amid increased fighting and instability in Chechnya, as well as neighboring Ingushetia and Dagestan, Russia has stepped up abductions and assassinations of Islamist leaders. The Islamists, in turn, have vowed to visit bloodshed on cities in the heart of Russia.

Now Russians are watching keenly to see how Moscow will respond. The public had largely ignored the rampant killings, disappearances and torture that beset its southern flank -- until it spilled into Moscow.